Either of two fat-soluble sterol-like compounds, ergocalciferol (vitamin D2) and activated cholecalciferol (vitamin D3). Vitamin D2 is formed from the irradiation of ergosterol, a plant sterol. However, vitamin D3 is normally manufactured in the skin, where ultraviolet light activates the compound 7-dehydrocholesterol. Vitamins D3 and D2 are about equal in activity in all mammals except New World monkeys and birds, in which vitamin D2 is approximately one-tenth as active as vitamin D3. See also Vitamin.
Vitamin D as acquired from the diet or produced in the skin is biologically inactive. It must be metabolized by the liver to produce 25-hydroxyvitamin D3. However, this compound is also biologically inactive under physiological circumstances and must be activated by the kidney to produce the final vitamin D hormone, 1,25-dihydroxyvitamin D3. This hormonal form of vitamin D plays an essential role in stimulating intestinal absorption of calcium and phosphorus, in the mobilization of calcium from bone, and in renal reabsorption of calcium. The function of vitamin D has been expanded beyond regulating plasma calcium and phosphorus levels, and hence healing the diseases of rickets and osteomalacia. It is now known that the vitamin D hormone controls parathyroid gland growth and production of the parathyroid hormone. It is an immunomodulator. Vitamin D hormone also appears to play a role in the regulation of insulin production or secretion. Finally, it is required for female reproduction. These new sites of action of vitamin D are under intense investigation. See also Hormone.
Vitamin D is largely absent from the food supply. It is found in large amounts in fish liver oils; cod liver oil has long been known to be an important source of vitamin D. Fortified foods are the major dietary source of vitamin D, but the major overall source is the production of vitamin D in skin by exposure to sunlight or ultraviolet irradiation. In winter months at temperate latitudes, insufficient amounts of vitamin D are produced in skin, and unless it is replaced by a dietary source, danger of insufficiency exists.
A deficiency of vitamin D in growing animals results in the disease rickets. A similar disease, osteomalacia, occurs in adult animals. By far the most serious disorder of vitamin D deficiency is the low-blood calcium levels which result in convulsions known as hypocalcemic tetany. Moderate deficiency of vitamin D may contribute to osteoporosis, especially in the elderly. See also Bone; Osteoporosis.
The recommended daily requirement for vitamin D3 is 10 micrograms or 400 international units (IU). Higher requirements are reported for the elderly and for rapidly growing adolescents: 20 μg or 800 IU per day. It is possible that the average requirement is lower than 10 μg per day. The exact absolute requirement has never been determined.